A chain-type door latch is known which has a support mounted on the door, another support secured to the doorjamb, and a flexible and inextensible element--normally a chain--anchored to the door support and releaseably engageable with the support on the doorjamb. Such an arrangement allows the door to be opened a limited distance so that a person on the inside of the door can see, talk to, and even receive small items from a person outside the door without having to open the door completely and allow the outside person in.
In a common variant on this type of latch a key-operable mechanism is provided on the doorjamb support so that after opening the door by its main lock a person equipped with a key can reach in through the narrow gap that the chain allows the door to open to actuate the key mechanism on the doorjamb and thereby release the end piece of the chain. Such a latch is typically added to a door to increase the security thereof, in particular in an apartment where the tenant fears that a key to the main apartment-door lock might have fallen into the wrong hands.
Finally it is known to provide chain-type latches of the two above-described types with an alarm which sounds whenever an excessive strain is exerted on the chain or other element extending between the door-mounted support and the doorjamb-mounted support. This arrangement has the advantage that if the latch is forced the alarm will sound to alert persons in nearby areas and if possible to scare away the person making the forced entry.
This last type of described system has the disadvantage, however, that the latch can be overcome without sounding the alarm through the simple expedient of cutting the flexible element extending between the door and the doorjamb, so that no strain is exerted on the chain or other element that would cause the alarm to be emitted. Another disadvantage of these systems is that during normal use an excessive force is sometimes exerted against the door, so that the alarm is emitted accidentally when in fact no cause for alarm exists.